Roto-Zip RZ-20 (has a right angle grinder, invaluable in this)
A set of pointy steel punches and one drift punch.
A pair of long channel-lock or vise grip pliers.
A 3lb hammer.
A drill press with split-point titanium nitride bits. I suspect that any good bits would do the trick.
Eye protection. Not optional.
Grind off the (flat) ends of the rivets sticking out from the torch-cut receiver fragment. That doesn't take much time, and then you can use a pair of pliers to peel off that bit of metal. This is the really easy part, since you don't care about cutting into the metal on this one.
Flip the trigger guard over, and start from the rounded heads of the rivets. Use the 3lb hammer and pointy steel punches to start a hole in the center of the rivets. Stop when you've opened up a decent hole, don't worry about the rivet head expanding. You actually kinda want it to do that.
Use the drill press to drill down through the center of the rivet. Because they're short, this is really easy. Go all the way through the rivet. I personally used the bit of the receiver removed from right above the trigger guard, as a buffer to protect the jaws of my drill press vise. As soon as you see that start vibrating down below the rivet, you know the bit is through and it's time to retract the press.
Take the right angle grinder, and slice a groove in the tops of the rivet heads. Pretend you're making a flat-head screwdriver groove. Don't go all the way down to the trigger guard; stop a little bit before then.
Now, the part that makes it easy. Take the pointy steel punch, and drive into the rivet heads from the sides, crushing them basically INTO the groove you just made. That decreases the inner diameter of the rivet, and it'll pull away the rivet from the walls of the shaft. You'll want to collapse the rivet on the head (rounded) side and also the other side, assuming that you have available rivet above the trigger guard plate.
Run the drill press through the center of the rivet again. You don't need move up to a larger bit, because you're just taking out the material that you crushed towards the center. Functionally, you're shrinking the rivet in the hole.
If the rivet doesn't pop out with a couple strikes from a drift punch, repeat the punch-and-drill steps until you've removed enough material that it does come out readily.
This doesn't mark up your trigger guard nearly as much as getting it really tight in a vise and whacking it until the rivets pop out. Once you've got the hang of it, this method goes pretty fast. The last set of trigger guard rivets I popped out this way took about 5 minutes apiece.
The rivet in the tail of the trigger guard is REALLY simple - because it doesn't have three friends helping it stay put, you can grind it down a little, then twist the rear until it works free.